1. Field of the Invention
This invention relates to a process for deposit a material with very high abrasion and corrosion resistance on a metal material or a conductive ceramic material, thereby providing a firm coating thereon. Particularly, it relates to a surface treating process for giving high abrasion and corrosion resistance to a metal mold, a tool, machine parts or the like.
2. Description of Related Art
The applicant of this application applied for a patent on a technique for giving corrosion and abrasion resistance by coating a surface of a metal material or the like by deposition by use of an electric discharge machining, and such technique is already well-known. The main point of such conventional technique is as follows.
(1) In one method, an electric discharge is performed in a working fluid by using an electrode which is formed by mixing and compressing powders of WC and Co. After a coating material is deposited on a workpiece, an electric discharge is carried out again by using another electrode, e.g. a copper electrode or a graphite electrode, for melting the coating material. Thus, the coating material is given higher hardness and better adhesion to the workpiece.
(2) In another surface treating method by electric discharge, an electrode is formed by compressing titanium (Ti). In an electric discharge using this electrode, Ti pyrochemically reacts with carbon generated from a working fluid which is thermally decomposed. Then, Ti becomes TiC (titanium carbide), that is a material of very high hardness, and is deposited on a workpiece or a base metal to form a coating thereon. At this time, a metal like Co (cobalt) which can become a binder is added to Ti as a compressed electrode material.
A conventional surface treating technique will be described hereafter referring to FIG. 1 and FIGS. 2a and 2b. In FIG. 1, step S1 shows a primary processing and step S2 shows a secondary processing. FIG. 2a shows the primary processing S1 and FIG. 2b shows the secondary processing S2. In the primary processing S1, an electric discharge machining is performed between a green compact electrode 13v of a mixture of WC-Co (tungsten-carbide-cobalt) and a workpiece 15 (a base metal S50C) in a working fluid, thereby depositing WC-Co on the workpiece 15. Here, the green compact electrode 13 is joined to a leading end of a copper electrode 11 to define a discharge electrode 10. Next, in the secondary processing S2, the deposited WC-Co layer 17 is re-fused by another electric discharge machining by use of a non-consumable electrode 21 which is hard to wear like a copper electrode.
A structure of the coating layer 17 obtained by the deposition in the primary processing S1 has a hardness of about Hv=1410 and contains many voids. However, the re-fusion in the secondary processing S2 makes the voids in the coating layer 17 disappear and improves the hardness up to Hv=1750 (see FIGS. 3a-3c).
In the above mentioned methods, the coating powders are deposited very well on a steel with high adhesion. It shows a hardness about 50% higher than that of a sintered hard metal of WC+Co or TiC+Co having the same component. For example, a hardness of a common hard metal tool of WC70+Co30 is Hv=850-950. On the other hand, if such a hard metal of the same component has its surface treated with the electric discharge processing, it shows a hardness of Hv=1710 after the secondary processing.
However, in the conventional methods, it is difficult to form the coating layer having strong adhesion to a surface of a sintered material, e.g. a hard metal bite. Moreover, adhesion strength of the coating layer has large unevenness.
That is, the coating layer adheres well to a steel surface, but hardly sticks to a hard metal surface or the like with the conventional methods. The reason is as follows. Here, main features of the present invention relate to a coating by deposition of Ti and its mixture, so that it is described about Ti why such a phenomenon takes place.
Ti is a metal whose fusing point is 1800.degree. C. and boiling point is 3000.degree. C. or more. Ti is covered with a thin and compact oxide film (Ti--O.sub.2) in the air at a normal temperature and chemically stable. That is like aluminum is covered with a compact oxide film Al.sub.2 O.sub.3. Then, if Ti powders are compressed into an electrode for use in the electric discharge machining (green compact electrode), the following phenomenon takes place.
When an electric discharge is generated between an electrode surface and a workpiece surface, the discharge point becomes a fusing point of a material thereat. At the same time, a working fluid (mineral oil) undergoes explosive decomposition by heat of vaporization. Then, the material at the discharge point is scattered since it is at a high temperature. The scattered material hits a counter electrode, namely, the workpiece surface to be processed. Usually, about 50% of the hitting material is deposited on the workpiece surface.
An electric discharge can be generated though Ti has a thin oxide film in the air. This is because the oxide film is very thin and easy to cause a dielectric breakdown. Namely, the electric discharge is generated by the dielectric breakdown. Then, if a voltage is made high or a distance is made short between the electrodes (discharge electrode and workpiece), potential gradient (V/cm) between the electrodes becomes high to bring forth dielectric breakdown, thereby generating an electric discharge.
This phenomenon can be understood by the fact that a corona discharge is generated at a high-tension transmission line or that a tunnel current flows through a thin oxide film. However, if the distance between the electrodes is made short in order to heighten the potential gradient, an electric discharge takes place and a fused metal swells on the electrode by the discharge pressure. If the swelling metal on one electrode touches with the facing other electrode before it separates from the one electrode, there arises a short circuit between the electrodes and the electric discharge stops. In short, the electric discharge becomes unstable. The applicants have already experienced that the electric discharge is unstable with respect to the Ti electrode or the Ti green compact electrode.
The hot titanium chemically reacts with carbon, which is generated from the decomposed working fluid, during Ti hits the workpiece and until the workpiece surface is covered with the hitting Ti and the first coating is covered with a next hitting Ti. A part of them becomes TiC. If the workpiece is made of a material easy to make an alloy with Ti like a steel and if its fusing point is relatively low compared with a hard metal or the like, Ti is fused well into the base metal (workpiece) or deposited on the base metal while adhering thereto when hitting it. For example, the steel has a fusing point of 1560.degree. C. and a boiling point of 2500.degree. C.
If the secondary processing is performed on the coating obtained by deposition by the same electrode or a different electrode while changing an electrode polarity or electric discharge conditions, the voids caused by the first deposition are crushed and disappear by re-fusion. Thus, it is possible to provide a deposited layer or a coating with high density. Such is described in the former application of the applicants. FIGS. 3a-3c are micrographs showing a structure of the deposited layer formed in the primary processing and a structure formed after the secondary processing.
However, in case the workpiece is a hard metal (sintered alloy of WC+Co, WC+Co+Ti) or the like, the coating of the Ti green compact is easy to be peeled off from the workpiece surface, even if it is deposited thereon. Namely, Ti is hardly deposited on the workpiece. This fact will be easily understood if considering a welding of metal materials. The steels can be welded by the arc welding. On the other hand, the hard metals cannot be welded by the arc welding. Moreover, the hard metal and the steel cannot be welded by the arc welding.
Still, if a surface of the steel is oxidized, an arc welding thereof is impossible. Therefore, it is common to use a flux for a welding rod or a welding wire to prevent oxidation. On the other hand, there is a material like aluminum which has difficulty in arc welding in the usual state even if its fusing point is low. This is because the surface of aluminum is always covered with a thin and compact film of an aluminum oxide in the air. It is known that welding is possible for the aluminum if the oxide film is destroyed by ultrasonic oscillation or the like.
The reason why Ti of the green compact electrode hitting the workpiece is not deposited on the surface of a hard metal is described hereafter in view of the above phenomenon in welding. It is thought that, since the surface of the Ti powders is covered with a thin oxide film (TiO.sub.2), such a film prevents the deposited layer from adhereing to the workpiece. Namely, the smaller a size of the Ti powder is, the larger a ratio of the powder surface area is, compared with a volume of the powder. Therefore, the ratio of the oxide on the powder surface increases.
A similar phenomenon takes place if a quantity of an oxidized surface increases or an oxide adhering to a workpiece largely acts in welding. The above fact can be explained as follows.
The ratio of the powder surface area to the powder volume is shown hereunder.
1) In case the shape of the powder is supposed to be a sphere: PA1 2) In case the shape of the powder is supposed to be a cube:
Surface Area: S=.pi..multidot.d.sup.2 PA2 Volume of Powder: V=.pi.d.sup.3 /6 (wherein d is a diameter of a powder.) PA2 Ratio of Surface Area to Volume: S/V=6/d PA2 Surface Area: S=6.multidot.d.sup.2 PA2 Volume of Powder: V=d.sup.3 (wherein d is a length of one side.) PA2 Ratio of Surface Area to Volume: S/V=6/d
From the above study, it is understood that, if the size of the powder is smaller, the ratio of the surface area to the volume increases. Therefore, in case the powder surface is closely covered with an oxide film or the like, the smaller the size of the powder is, the more the processing is influenced by the oxide film.
In addition, it is thought that the high fusing point of the hard metal makes welding difficult. This is because the high fusing point makes a fused portion of the hard metal difficult to flow in welding. To the contrary, a fused portion of a steel is easy to flow in welding.
Taking it into account that the oxide layer on the powder surface hinders the deposited layer from fusing and adhering to the workpiece, the compressed powders are easily influenced by an oxide and, if the powder size is smaller, such influence by the oxide becomes larger. Compared with that, in the case of a solid metal titanium electrode, the ratio of an oxide layer to the surface is small. Therefore, it is possible to coat the surface with a metal Ti electrode though it is inefficient.
Ti is deposited on the workpiece rather well in case of the solid Ti electrode. Ti is deposited rather well, too, in case of the electrode sintered or temporarily sintered in a vacuum furnace or the like. However, a depositing quantity (thickness) by the Ti solid electrode or the Ti sintered electrode is small and their adhesion strength is lower compared with a TiH.sub.2 green compact electrode described later. Namely, it is supposed that an obstruction factor by an oxide remains unsolved.
As obvious from the above description, in the conventional surface treating method using the electric discharge, the material powders of the green compact electrode of Ti or the like is closely covered with the oxide film (TiO.sub.2). Therefore, it is understood that, even if oxygen separates, in part, from the powder surface in the electric discharge, the oxide film still prevents the powdered metal forming the electrode from being deposited on the workpiece surface and fusing with the workpiece metal. Moreover, the thermal decomposition temperature of TiO.sub.2 is very high (1800.degree. C.). Thus, when the metal powders of the electrode are scattered due to the electric discharge pressure, many powders hit the workpiece surface in the form of TiO.sub.2. In addition, it is necessary to make clearance between the electrodes for generating the electric discharge narrower, since the oxide film makes the electric discharge difficult to generate. Thus, short circuits increase in the surface treating processing. Such being the case, it is understood that the oxide film deteriorates the workpiece surface and affects the processing efficiency.